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I am using the OPA842 in a traditional trans-impedance configuration with a BF862 as input buffer (included in the feedback-loop). Gain is 20dB up to around 10MHz.
I have measured IP3 on this amplifier with f1:1.1MHz and f2:1.11Mhz and found that the slope of the third-order products is far less than 3 times the slope of the fundamentals. In fact the slope was only 1.3 times (in dB), which means that the resulting IP3 is highly depending of the level of the input signal.
Is this a normal behaviour?
If so, it seems to make IP3 a meaningless figure or at least you have to specify the test conditions.
Looking forward to seeing your comments on this. Thanks in advance.
Best regards
Karsten
Hi Samir,
I used a ZNB 8 network analyzer for the test as described in the attached file. It goes down the 9kHz so the issue is not a bandwidth limitation of the instrument.
I have just noticed that at high input port to the amplifier the slope seems more "normal", unfortunately I dont have the instrument available any more, so I cannot decide if it is a test error.
Once again thanks for your assistance.
Best regards